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13-1
CHAPTER 13
INTERFACING WITH EXTERNAL MEMORY
The device can interface with a variety of external memory devices. Six chip-selects can be indi-
vidually programmed for bus width, the number of wait states, and a multiplexed or demulti-
plexed address/data bus. Other features of the external memory interface include ready control
for inserting additional wait states, a bus-hold protocol that enables external devices to take con-
trol of the bus, and two write-control modes for writing words and bytes to memory. These fea-
tures provide a great deal of flexibility when interfacing with external memory devices.
In addition to describing the signals and registers related to external memory, this chapter discuss-
es the process of fetching the chip configuration bytes and configuring the external bus. It also
provides examples of external memory configurations and chip-select setup.
13.1 INTERNAL AND EXTERNAL ADDRESSES
The address that external devices see is different from the address that the device generates inter-
nally. Internally, the device has 24 address lines, but only the lower 20 address lines (A19:0)
are implemented with external pins. The absence of the upper four address bits at the external pins
causes different internal addresses to have the same external address. For example, the internal
addresses FF2080H, 7F2080H, and 0F2080H all appear at the 20 external pins as F2080H. The
upper nibble of the internal address has no effect on the external address.
The address seen by an external device also depends on the number of address lines that the ex-
ternal system uses. If the address on the external pins (A19:0) is F2080H, and only A17:0 are con-
nected to the external device, the external device sees 32080H. The upper four address lines
(A19:16) are implemented by the EPORT. Table 13-1 shows how the external address depends
on the number of EPORT lines used to address the external device.
Table 13-1. Example of Internal and External Addresses
EPORT Lines
Connected to the
External Device
Internal Address
Address on the
Device Pins
Address Seen by
External Device
A16
x
F2080H F2080H 12080H
A17:16
x
F2080H F2080H 32080H
A18:16
x
F2080H F2080H 72080H
A19:16
x
F2080H F2080H F2080H